In order to provide an adequate supply of liquid hydrocarbons for use as synfuels or chemical feedstocks, various processes have been developed for converting coal and natural gas to gasoline, distillate and lubricants. A substantial body of technology has grown to provide oxygenated intermediates, especially methanol. Large scale plants can convert methanol or similar aliphatic oxygenates to liquid fuels, especially gasoline. However, the demand for heavier hydrocarbons has led to the development of processes for increasing the yield of gasoline and diesel fuel by multi-stage techniques.
Recent developments in zeolite catalysts and hydrocarbon conversion processes have created interest in utilizing olefinic feedstocks for producing C.sub.5.sup.+ gasoline, diesel fuel, etc. In addition to the basic work derived from ZSM-5 type zeolite catalysts, a number of discoveries have contributed to the development of a new industrial process, known as Mobil Olefins to Gasoline/Distillate ("MOGD"). This process has significance as a safe, environmentally acceptable technique for utilizing feedstocks that contain lower olefins, especially C.sub.2 -C.sub.5 alkenes. This process may supplant conventional alkylation units. In Plank, Rosinski and Givens U.S. Pat. No(s). 3,960,978, and 4,021,502, disclose conversion of C.sub.2 -C.sub.5 olefins alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al have also contributed improved processing techniques to the MOGD system, as in U.S. Pat. No(s). 4,150,062, 4,211,640 and 4,227,992. The above-identified disclosures are incorporated herein by reference.
Conversion of lower olefins, especially propene and butenes, over ZSM-5 is effective at moderately elevated temperatures and pressures. The conversion products are sought as liquid fuels, especially the C.sub.5.sup.+ aliphatic and aromatic hydrocarbons. Olefinic gasoline is produced in good yield by the MOGD process and may be recovered as a product or recycled to the reactor system for further conversion to distillate-range products. Operating details for typical MOGD units are disclosed in Owen et al U.S. Pat. No(s). 4,445,031 and 4,456,779, and Tabak U.S. Pat. No. 4,433,185, incorporated herein by reference.
In addition to their use as shape selective oligomerization catalysts, the medium pore ZSM-5 type catalysts are useful for converting methanol and other lower aliphatic alcohols or corresponding ethers to olefins. Particular interest has been directed to a catalytic process ("MTO") for converting low cost methanol to valuable hydrocarbons rich in ethene and C.sub.3.sup.+ alkenes. Various processes are described in Batter et al U.S. Pat. No. 3,894,107, Chang et al U.S. Pat. No. 3,928,483, Lago U.S. Pat. No. 4,025,571, Daviduk et al U.S. Pat. No. 4,423,274, and Young U.S. Pat. No. 4,433,189, incorporated herein by "reference. It is generally known that the MTO process can be optimized to produce a major fraction of C.sub.2 -C.sub.4 olefins. "Prior process proposals have included a separation section to recover ethene and other gases from by-product water and C.sub.5.sup.+ hydrocarbon liquids. The oligomerization process conditions which favor the production of C.sub.10 -C.sub.20 and higher aliphatics tend to convert only a small portion of ethene as compared to C.sub.3.sup.+ olefins.
The Gould et al U.S. Pat. No. 4,579,999 discloses an integrated process for the conversion of methanol to gasoline and distillate. In a primary catalytic stage (MTO) methanol is contacted with zeolite catalyst to produce C.sub.2 -C.sub.4 olefins and C.sub.5.sup.+ hydrocarbons. In a secondary catalytic stage (MOGD) containing an oligomerization catalyst comprising medium pore shape selective acidic zeolite at increased pressure, a C.sub.3.sup.+ olefins stream from the primary stage is converted to gasoline and/or distillate liquids.
The Harandi et al U.S. Pat. No. 4,899,002 discloses a process for the increased production of olefinic gasoline, which comprises the integration of oxygenates to olefin (MTO) conversion with olefin to gasoline conversion under moderate severity conditions. The product of the olefins to gasoline conversion is passed to an olefin to gasoline and distillate (MOGD) conversion zone for distillate production.
The methanol to olefin process (MTO) operates at high temperature and moderate pressure and high catalyst contact time in order to obtain efficient conversion of the methanol to olefins. These process conditions, however, produce an undesirable amount of aromatics and C.sub.2.sup.- light gas stream and require a large investment in plant equipment.
The olefins to gasoline and distillate process (MOGD) operates at moderate temperatures and elevated pressures to produce olefinic gasoline and distillate products. When the conventional MTO process effluent is used as a feed to the MOGD process, the aromatic hydrocarbons produced in the MTO unit are desirably separated and a relatively large volume of MTO product effluent has to be cooled and treated to separate a C.sub.2.sup.- light gas stream, which is unreactive, except for ethene which is reactive to only a small degree, in the MOGD reactor, and the remaining hydrocarbon stream has to be pressurized to the substantially higher pressure used in the MOGD reactor.
The problems to be solved were to reduce the overall size and investment in the MTO reactor, reduce the amount of the methanol feed fed to the MTO reactor in order that the process could be carried out under lower severity operating conditions which improves selectivity to not produce aromatics and not produce large amounts of C.sub.2.sup.- light gas. At the same time it was desired to maintain the total effective amount of the methanol feed converted to olefins and to improve the overall selectivity of the MTO/MOGD process to produce more olefinic gasoline and distillates.